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At the time of the Big Bang, 13.8 billion years ago, each particle of matter is thought to have been produced together with an equivalent of antimatter of opposite electrical charge. But there is much more matter than antimatter in the current universe. Why this is the case is one of the biggest questions in physics.
The answer may lie, at least in part, in particles called neutrinos, which lack electric charge, are nearly massless, and change their identities or “swing” from one of three types to another as they travel through space. If neutrinos oscillated differently from their antimatter equivalents, antineutrinos, they could help explain the matter-antimatter imbalance in the universe.
Experiments around the world, such as the NOvA experiment in the United States, are investigating this possibility, as are next-generation experiments including DUNE. In these long baseline neutrino oscillation experiments, a neutrino beam is measured after it has traveled a long distance on the long baseline. The experiment is then performed with an antineutrino beam and the result is compared with that of the neutrino beam to see if the two twin particles oscillate similarly or differently.
This comparison depends on an estimate of the number of neutrinos in the neutrino and antineutrino beams before they travel. These beams are produced by firing beams of protons at stationary targets. Interactions with the target create more hadrons, which are focused via magnetic “horns” and directed into long tunnels where they transform into neutrinos and other particles. But in this multi-step process, it is not easy to calculate the particle content of the resulting beams, including the number of neutrinos they contain, which directly depends on the interactions of the proton with the target.
Enter the NA61 experiment at CERN, also known as SHINE. Using high-energy proton beams from the Super Proton Synchrotron and appropriate targets, the experiment can recreate the related proto-target interactions. NA61/SHINE has previously made measurements of electrically charged hadrons that are produced in interactions and produce neutrinos. These measurements have helped to improve the estimates of neutrino beam content used in existing long baseline experiments.
The NA61/SHINE collaboration has now released new hadron measurements that will help further improve these estimates. This time, using a proton beam with an energy of 120 GeV and a carbon target, the collaboration measured three types of electrically neutral hadrons that decay into charged hadrons that produce neutrinos.
This 120 GeV protoncarbon interaction is used to produce the NOvA neutrino beam, and will probably also be used to create the DUNE beam. Estimates of the numbers of the different neutrino-producing neutral hadrons that the interaction produces are based on computer simulations, the result of which varies significantly depending on the underlying physical details.
“Until now, simulations for neutrino experiments using this interaction have relied on uncertain extrapolations from previous measurements with different energies and target nuclei. This new direct measurement of particle production from 120 GeV protons on carbon reduces the need of these extrapolations”. explains NA61/SHINE deputy spokesperson Eric Zimmerman.
The document is published in the journal Physical review D.
H. Adhikary et al, Measurements of KS0 , , and production in 120 GeV/c p+C interactions, Physical review D (2023). DOI: 10.1103/PhysRevD.107.072004
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